1. Field of the Invention
The present invention relates to an optical scanning system, and in particular to a polygon mirror scanner with poorer precision that can distribute its velocity jitter, further can improve the quality and lower the cost of the optical scanning system.
2. Description of Prior Art
For equipment that employs an optical scanning system, such as a laser printer, the requirements of the velocity jitter (i.e., speed variation) of an optical scanner increases as the resolution degree is improved. In addition, the accuracy of a scanning velocity jitter also increases as the resolution degree improves.
Besides, the standard for the velocity jitter and the cost become increasing high as a printing paper is enlarged or resolution degree improves.
Nowadays, if the standard of the velocity jitter is too strict, the polygon mirror scanner technique will be difficult to meet specifications.
FIG. 1 illustrates a schematic diagram of a prior art optical scanner system employing a laser printer. The prior art optical scanner system is suited for use in the electronic control system 10. A light beam is emitted by a modulated laser diode 12, and then impinges on a photoconductive drum 14. The prior art optical scanner system primarily comprises a collimating lens 20, a cylindrical lens 30, a polygon mirror 40 and an optical scanning lens 50. The light beam of laser diode 12 having image information which is a series of modulating signals. The collimating lens 20 makes all the light beam parallel. The parallel light beam as a whole is considered as a modulating signal beam 11. The cylindrical lens 30 focuses the modulation light beam 11 which is passed through the collimating lens 20 and the cylindrical lens 30. Then, the light beam impinges on a rotating polygon mirror 40. The rotating polygon mirror 40 is driven to rotate by a motor at constant speed. The polygon mirror 40 is provided with a plurality of identical flat mirrors, and, thus, the light beam incident upon one of the mirror surfaces of the polygon mirror 40 is reflected so that the reflected light beam now passes through an optical scanning lens 50, which serves as a linear focusing lens. Then the light beam is reflected by a flat mirror 16 to impinge upon a photoconductive drum 14.
Besides, the synchronizing light beam 22 passes through the optical scanning lens 50, a reflecting mirror 62, a focusing lens 64, and an optical fiber 66 and then impinges on a photodiode 68. The photodiode 68 receives the light beam 22 and transfers it to an electronic signal. Then, the electronic signal passes through the electronic control system 10. The electric signal is used as a starting synchronizing signal of each line printing signal. Because the position of the optical scanning lens 50, the reflecting mirror 62, the focusing lens 64, the optical fiber 66 and the photodiode 68 are fixed, the light beam position can be treated as a reference point, so as to lessen the influence caused by the angle inaccuracy and velocity jitter of the polygon mirror 40, and let the starting points of all printing lines be the same. Therefore, the synchronizing light beam 22 can be treated as the starting synchronizing signal of each line printing signal.
FIG. 2a-2d are comparative diagrams of the effect of the velocity jitter of the prior art optical scanner system in FIG. 1 on the printing velocity (or printing length), in which FIG. 2a shows a synchronizing signal, FIG. 2b shows a normal printing velocity, FIG. 2c shows that the scanning velocity becomes faster and the printing length becomes longer when the velocity jitter is positive, and FIG. 2d shows that the scanning velocity becomes slower and the printing length becomes shorter when the velocity jitter is negative. When the rotating velocity of the polygon mirror scanner 40 is a constant, i.e., the scanning velocity of every edge of the polygon mirror scanner is the same, the printing length of every line will also be the same. However, if the polygon mirror scanner 40 has a phenomenon of velocity jitter, there will be a change to the scanning velocity. Although all the starting printing points are at the same position 80 as shown in FIG. 2, when the velocity jitter is positive, the scanning velocity becomes faster, the printing length becomes longer, and produce a position error E. When the velocity jitter is negative, the scanning velocity becomes slower, the printing length becomes shorter, and is produced a position error E'. Therefore, when the polygon mirror scanner 40 has a phenomenon of velocity jitter, the printing velocity will be sometimes faster and sometimes slower, this will cause a bad effect on a printed graph or character. Although it is impossible for the velocity jitter to be zero, it can be small enough such that the position error is approximately to be zero and therefore there is almost no influence to the printing quantity. However, the smaller the velocity jitter is, the higher the cost becomes.